The classification of herpesviruses has been updated as a result of recommendations made to the International Committee on Taxonomy of Viruses (ICTV; http://www.ictvonline.org) by the Herpesviridae Study Group as it was constituted in the period 2002–2005. The purpose of this report is to convey the taxonomic changes.

Morphologically, herpesviruses are distinct from all other viruses. A linear, double-stranded DNA genome of 125–290 kbp is contained within a T = 16 icosahedral capsid, which is surrounded by a proteinaceous matrix dubbed the tegument and then by a lipid envelope containing membrane-associated proteins [18]. Genetically, herpesviruses fall into three distinct groupings that are related only tenuously to each other [4, 5, 7]. These groupings consist of viruses of mammals, birds and reptiles, viruses of fish and frogs, and a single virus of bivalves. Of the few proteins that are detectably conserved in sequence among the three groupings, none has homologues that are found only in herpesviruses. The protein that comes nearest to being herpesvirus-specific is the putative ATPase subunit of the terminase (a complex that is responsible for packaging virus DNA into nascent capsids), which is conserved in all herpesviruses and, to a lesser degree, T4-like bacteriophages in the family Myoviridae. The taxonomic outcome of the apparently large genetic distances among the groupings is that the former family Herpesviridae [6], which contained all herpesviruses, has now been divided into three families that comprise the new order Herpesvirales. The revised family Herpesviridae retains the viruses of mammals, birds and reptiles, the new family Alloherpesviridae (from ἄλλος—other, different) incorporates the fish and frog viruses, and the new family Malacoherpesviridae (from μαλακός—soft; μαλάκιον—mollusc) contains the bivalve virus. The revised classification is shown in Table 1.

Table 1 The order Herpesvirales
Full size table

Three new genera have been created in the family Herpesviridae. One is the genus Proboscivirus (from προβoσκίς or proboscis: elephant’s trunk), which recognizes a distinct lineage of the subfamily Betaherpesvirinae containing the new species Elephantid herpesvirus 1 [9, 16, 19]. The others are the genera Macavirus (sigla from malignant catarrhal fever) and Percavirus (sigla from perissodactyl and carnivore), which form two lineages in the subfamily Gammaherpesvirinae that are separable from the established genera [14, 15]. The genus Macavirus contains the established species Alcelaphine herpesvirus 1, Alcelaphine herpesvirus 2, Hippotragine herpesvirus 1 and Ovine herpesvirus 2, which were transferred from the genus Rhadinovirus, plus the new species Bovine herpesvirus 6, Caprine herpesvirus 2, Suid herpesvirus 3, Suid herpesvirus 4 and Suid herpesvirus 5 [1, 2, 23]. The genus Percavirus contains the established species Equid herpesvirus 2, Equid herpesvirus 5 and Mustelid herpesvirus 1, which were also transferred from the genus Rhadinovirus. Other new species include Columbid herpesvirus 1 and Psittacid herpesvirus 1 in the genera Mardivirus and Iltovirus, respectively, of the subfamily Alphaherpesvirinae [8, 22]. In addition, several new species were classified at the level of subfamily or family, with assignment to genera awaiting further data. These include the reptilian species Chelonid herpesvirus 5 and Chelonid herpesvirus 6 in the subfamily Alphaherpesvirinae [3, 11, 13, 17, 26], Phocid herpesvirus 2 in the subfamily Gammaherpesvirinae [10, 12], and Iguanid herpesvirus 2 in the family Herpesviridae [13, 25].

Herpesvirus species are named after a taxon of the host that in its natural setting harbours the virus [20, 21]. The default host taxon employed is that of family, and, except for the species of humans, the name ends in ‘-id’. Owing to their larger numbers, species from nonhuman primates and the family Bovidae have been designated by host subfamily, with the name ending in ‘-ine’. In the revised taxonomy, species from nonhuman primates have been renamed by host genus, with the name again ending in ‘-ine’. This has resulted in name changes for several Old World and one New World nonhuman primate herpesvirus species, and has reduced the very large number of species that previously incorporated the Cercopithecine prefix. The new names are correlated to the former names in Table 1.

The established genus Ictalurivirus has been transferred to the family Alloherpesviridae, and the new species Cyprinid herpesvirus 3 been added to the family [24]. The new genus Ostreavirus (from ὄστρεον or ostrea—oyster) has been created in the family Malacoherpesviridae, containing the single known species of an invertebrate host, Ostreid herpesvirus 1 [7].

The need to balance logic and utility makes taxonomy a challenging process, and matters are often not straightforward. In addition to keeping abreast of new viruses as they are discovered, current discussions in the Herpesviridae (now Herpesvirales) Study Group (chaired by P.E. Pellett) include updating the herpesvirus species definition and the methods used for taxonomic assignment, in order to accommodate the well-established value of sequence-based phylogenetic relationships, and defining a pathway to species status for the large and increasing number of herpesviruses that have been detected only by PCR.